Methods for reusing active materials from cathode waste

Abstract

A method is provided for recovering and reusing active materials from cathode waste. The cathode active material reuse method of the present invention obtains reusable active materials through the following steps: (a) heat-treating cathode waste containing a lithium transition metal composite oxide cathode active material layer, all disposed on a current collector, in air, thereby thermally decomposing the binder and conductive material contained in the active material layer, thereby separating the current collector from the active material layer and recovering the active material contained in the active material layer; (b) washing the recovered active material with a washing solution; (c) adding a lithium precursor to the washed active material and annealing the active material; and (d) selectively surface-coating the annealed active material, wherein the amount of lithium precursor added in step (c) is determined such that the amount of residual lithium compounds in the reusable active material is from 0.0001 wt% to 1.2 wt%.

Description

Technical Field

[0001] This disclosure relates to a method for recycling resources during the manufacture of lithium-ion secondary batteries. In particular, this disclosure relates to a method for recovering and reusing positive electrode active materials from positive electrode waste generated during the manufacture of lithium-ion secondary batteries or from discarded lithium-ion secondary batteries after use. This application claims priority to Korean Patent Application 10-2020-0106083, filed in Korea on August 24, 2020, the disclosure of which is incorporated herein by reference. Background Technology

[0002] Rechargeable lithium-ion batteries have gained attention as an alternative to fossil fuels. While primarily used in traditional handheld devices such as mobile phones, cameras, and power tools, their applications have recently expanded to electric vehicles (EVs, HEVs, and PHEVs), high-capacity energy storage systems (ESS), and uninterruptible power supply systems (UPS).

[0003] Lithium-ion rechargeable batteries comprise: electrode assemblies, in which the unit cells have a structure in which a positive electrode plate coated with active material on a current collector and a negative electrode plate are arranged with a separator sandwiched between them; and an external material (i.e., the battery casing) that seals and houses the electrode assemblies with the electrolyte. The positive electrode active material of lithium-ion rechargeable batteries primarily utilizes lithium-based oxides, while the negative electrode active material utilizes carbon materials. Lithium-based oxides contain metals such as cobalt, nickel, or manganese. In particular, cobalt, nickel, and manganese are very expensive precious metals. Cobalt, in particular, is a strategic metal, and every country in the world pays special attention to its supply and demand. Due to the limited quantity produced by cobalt countries, it is considered a metal whose supply and demand are unstable globally. If the supply and demand of raw materials for strategic metals become unbalanced, the price of these raw materials is highly likely to rise.

[0004] Traditionally, research has primarily focused on recovering and recycling these precious metals from lithium-ion batteries discarded at the end of their service life (waste batteries). However, recovering resources from waste discarded after punching out the positive electrode plates, or from positive electrode plates that are defective in the process, is a more preferable approach, in addition to waste batteries.

[0005] Currently, in the manufacture of lithium secondary batteries, such as Figure 1As shown, a positive electrode plate 30 is manufactured by forming a positive electrode active material layer 20. In this layer 20, a long sheet-type positive electrode current collector 10, such as aluminum (Al) foil, is coated with a positive electrode slurry. This slurry contains a positive electrode active material, a conductive material, a binder, a solvent, etc., and the positive electrode plate 40 is then punched to a specific size. The remaining portion after punching is discarded as positive electrode waste 50. If the positive electrode active material can be recovered from the positive electrode waste 50 and reused, it would be highly desirable from both an industrial economic and environmental perspective.

[0006] Traditionally, the method for recovering positive electrode active materials in most cases involves dissolving the positive electrode in hydrochloric acid, sulfuric acid, nitric acid, etc., and then extracting active material elements such as cobalt, nickel, and manganese. These active material elements are then reused as raw materials for synthesizing positive electrode active materials. However, the method of extracting active material elements using acids has the disadvantage that the process of using pure raw materials or recovering pure raw materials is not environmentally friendly and requires neutralization and wastewater treatment processes, which increases process costs. In addition, this method also has the disadvantage that lithium, one of the main elements of positive electrode active materials, may not be able to be recovered. To overcome these disadvantages, a method is needed that does not dissolve the positive electrode active material and does not extract the active material in elemental form, but directly reuses the active material.

[0007] Furthermore, a method is needed to minimize the loss of constituent elements such as lithium during the process of obtaining reusable active materials. The loss of constituent elements such as lithium must be prevented as much as possible so as not to result in a significant difference in composition from fresh, unused active materials. By doing so, the process of adding insufficient components can be minimized. Summary of the Invention

[0008] Technical issues

[0009] This disclosure aims to address the problems of the prior art, and therefore, this disclosure aims to provide a method for recovering and reusing active materials from cathode waste.

[0010] Technical solution

[0011] In one aspect of this disclosure, a method for reusing positive electrode active material is provided, the method comprising the following steps: (a) thermally decomposing the binder and conductive material in the active material layer by heat-treating a positive electrode waste comprising a lithium composite transition metal oxide positive electrode active material layer on a current collector in air, separating the current collector from the active material layer, and recovering the active material in the active material layer; (b) washing the recovered active material with a cleaning solution; (c) adding a lithium precursor to the washed active material and annealing the active material; and (d) obtaining reusable active material by selectively surface-coating the annealed active material, wherein the amount of lithium precursor added in step (c) is determined such that the amount of residual lithium compound in the reusable active material is from 0.0001 wt% to 1.2 wt%.

[0012] Heat treatment can be carried out at 300°C to 650°C, especially for 10 minutes to 24 hours.

[0013] The heat treatment can be carried out at 550°C for 30 minutes at a temperature rise rate of 5°C / minute.

[0014] The cleaning solution may be water. Alternatively, the cleaning solution may be an aqueous solution of a lithium compound that is alkaline in its aqueous state. The aqueous solution of the lithium compound may be prepared to contain greater than 0% and equal to or less than 15% lithium compound, and preferably uses LiOH. Washing can be carried out within one week, preferably within one day, and more preferably within one hour.

[0015] Washing can be performed by immersing the recovered active material in the aqueous solution of the lithium compound while simultaneously stirring the recovered active material.

[0016] Lithium precursors may include at least one of LiOH, Li2CO3, LiNO3, and Li2O.

[0017] The lithium precursor can be added at a molar ratio of 0.09 to 0.12 relative to a 1:1 molar ratio of lithium to other metals.

[0018] For example, if drying is not performed after washing, the lithium precursor can be added in step (c) by mixing the washed active material in a lithium precursor solution and spray-drying the active material. Furthermore, the spray-drying temperature is preferably between 100°C and 300°C.

[0019] The annealing can be carried out in air at 400°C to 1000°C.

[0020] The annealing temperature can exceed the melting point of the lithium precursor.

[0021] The active material in the active material layer may be recovered in the form of powder, and the carbon component generated by carbonization of the binder or the conductive material may not remain on the surface.

[0022] Performing the surface coating may include coating at least one of a metal, an organometal, and a carbon component on the surface by a solid or liquid method, and then performing heat treatment at 100 °C to 1200 °C.

[0023] The reusable active material may be represented by Chemical Formula 1 below.

[0024] [Chemical Formula 1]

[0025] Li a Ni x Mn y Co z M w O 2+δ

[0026] (In Chemical Formula 1 above, M includes at least one selected from the group consisting of B, W, Al, Ti, and Mg, 1 < a ≤ 1.1, 0 ≤ x < 0.95, 0 ≤ y < 0.8, 0 ≤ z < 1.0, 0 ≤ w ≤ 0.1, -0.02 ≤ δ ≤ 0.02, and x + y + z + w = 1.)

[0027] The reusable active material may include fluorine (F) having a content equal to or less than 100 ppm.

[0028] Advantageous Effects

[0029] According to the present disclosure, waste positive electrode active materials (such as positive electrode waste generated in the manufacturing process of lithium secondary batteries) can be reused without using an acid, and thus are eco-friendly. The method according to the present disclosure does not require a neutralization process or a wastewater treatment process, thereby alleviating environmental problems and reducing processing costs.

[0030] According to the present disclosure, the positive electrode active material can be recovered without non-recyclable metal elements. Since the current collector is not dissolved, the current collector can also be recovered. The method can directly reuse the active material recovered in the form of powder, rather than extracting the active material elements and resynthesizing the active material elements as raw materials into the positive electrode active material, and thus is economical.

[0031] According to the present disclosure, toxic and explosive solvents such as NMP, DMC, acetone, and methanol are not used, and thus it is safe, and simple processes such as heat treatment, washing, and annealing are employed, and thus it is easy to manage the process and is suitable for mass production.

[0032] According to this disclosure, the electrochemical properties of the recovered active material will not deteriorate, and good resistance and capacity characteristics can be achieved.

[0033] In particular, according to this disclosure, lithium loss in the active material can be minimized in the heat treatment process for separating the current collector and the washing process (e.g., removing residues) for surface modification. By optimizing the heat treatment process, lithium loss can be minimized, as can the amount of current collector reacting with or precipitating from the active material through heat treatment. Since lithium loss is minimized during the process of obtaining the reusable active material, the composition of the fresh active material is not significantly different from that of the reusable active material. Insufficient lithium is added through an additional lithium precursor addition process. In this regard, the amount of precursor added is determined taking into account the amount of residual lithium compounds, which is therefore preferred in terms of process and cost.

[0034] According to this disclosure, capacity and cycle characteristics can be improved by optimizing the recycling process by limiting the amount of residual lithium compounds. Even if the amount of lithium precursor added is equal to the amount lost, errors can still occur due to analyses such as ICP. Furthermore, even if the capacity has been restored to the same level as the original fresh cathode active material, the defect rate of the battery cell may increase if a large amount of residual lithium compounds remain, because the lithium compounds react with the electrolyte, generating side reactions or gases in the battery cell. In this invention, the addition of lithium precursors is intended to limit the content of residual lithium compounds, thus solving these problems.

[0035] According to another aspect of this disclosure, since the active material to be reused can be re-granulated by spray drying, even if the particles are broken due to rolling in the previous process, the particle size and specific surface area can be improved. Furthermore, if the washed active material is mixed with a lithium precursor solution and spray-dried, the lithium precursor can be replenished while the active material is being re-granulated, thus simplifying the process and providing the advantage of forming a continuous process with the preceding washing step. Attached Figure Description

[0036] The accompanying drawings illustrate preferred embodiments of the present disclosure and, together with the foregoing disclosure, help to further understand the technical features of the present disclosure. Therefore, the present disclosure is not to be construed as limited to the drawings.

[0037] Figure 1 This is a diagram showing the waste positive electrode material discarded after the positive electrode plate is punched out from the positive electrode sheet.

[0038] Figure 2 This is a flowchart of a method for reusing active materials according to this disclosure.

[0039] Figures 3 to 5The cell evaluation results using the active materials of the embodiments and comparative examples are shown, wherein Figure 3 The initial charge / discharge characteristics are shown. Figure 4 yes Figure 3 Enlarged images of certain time periods, and Figure 5 The C-rate characteristics are shown.

[0040] Figure 6 This is a graph showing the results of pH titration. Detailed Implementation

[0041] The preferred embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. Before the description, it should be understood that the terminology used in this specification and the appended claims should not be construed as limited to its general and dictionary meanings, but rather should be interpreted according to the meanings and concepts corresponding to the technical aspects of this disclosure, based on the principle of allowing the inventors to appropriately define the terms to obtain the best interpretation. Therefore, the descriptions presented herein are merely preferred embodiments for illustrative purposes and are not intended to limit the scope of this disclosure; thus, it should be understood that other equivalents and variations can be made thereto without departing from the scope of this disclosure.

[0042] In the following description, reference is made to the accompanying drawings, which form a part of this document. The illustrative embodiments described in the detailed description, drawings, and claims are not intended to be limiting. Other embodiments may be utilized and other changes may be made without departing from the spirit and scope of the subject matter set forth herein. It will be readily understood that the various aspects of this disclosure, as generally described herein and illustrated in the figures, can be arranged, substituted, combined, separated, and designed into a variety of different configurations, all of which are expressly contemplated herein.

[0043] Unless otherwise defined, all technical and scientific terms used herein generally have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains.

[0044] This disclosure should not be limited to the specific embodiments described herein, which are intended to illustrate various aspects. It will be apparent to those skilled in the art that many variations and modifications can be made without departing from the spirit and scope of this disclosure. In addition to the methods and apparatuses listed herein, functionally equivalent methods and apparatuses within the scope of this disclosure will be apparent to those skilled in the art based on the foregoing description. Such variations and modifications should fall within the scope of the appended claims. This disclosure is limited only by the terms of the appended claims and the scope of all their equivalents. It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

[0045] In the case of conventional active material recycling processes, the main purpose is to extract precious metals (nickel, cobalt, manganese, etc.) from the active materials of lithium secondary batteries whose properties deteriorate after use and to resynthesize the active materials. However, the difference of this disclosure is that the active materials are recovered from the positive electrode waste generated during the manufacturing process of lithium secondary batteries.

[0046] Furthermore, in known active material recycling processes, the addition of chemical methods—such as acid / alkali dissolution or melting with reduction / additives to extract precious metals and then manufacturing them into metals (direct reduction method) or resynthesizing active materials—further complicates the process and incurs economic costs. However, this disclosure relates to a method for directly reusing positive electrode active materials without dissolving them.

[0047] To directly reuse the positive electrode active material, a method for removing the current collector from the positive electrode is needed. This can be achieved through methods such as high-temperature heat treatment to remove the binder, melting the binder with a solvent, completely melting the current collector, and selecting the active material through dry grinding and sieving.

[0048] Solvent stability is crucial for dissolving binders using solvents. While NMP is the most effective solvent, it suffers from toxicity and high cost. Furthermore, it requires solvent recovery processes, such as reprocessing waste solvents. Melting the current collector is cheaper than using solvents. However, it poses an explosion hazard due to the difficulty in removing foreign matter from the surface of the reused active material and the generation of hydrogen gas during the current collector removal process. Complete separation of the current collector from the active material is difficult through dry grinding and sieving. The particle size distribution of the active material changes during the grinding process, and the binder is difficult to remove, resulting in deterioration of the reused battery's characteristics.

[0049] In this disclosure, the active material and the current collector are separated by high-temperature heat treatment. In particular, since the heat treatment is carried out in air, no specialized equipment configuration is required, and because it is a relatively simple process requiring only heating, it is advantageous for large-scale production and commercialization. However, foreign matter should not remain on the surface of the reusable active material. This disclosure even proposes a step for removing foreign matter from the surface of the reusable active material.

[0050] In the following text, refer to Figure 2 A method for reusing active materials according to one embodiment of the present disclosure is described. Figure 2 This is a flowchart of a method for reusing active materials according to this disclosure.

[0051] refer to Figure 2 First, waste positive electrode material is prepared (step S10).

[0052] As referenced above Figure 1 The cathode waste can be the residue remaining after manufacturing a cathode sheet including a lithium composite transition metal oxide cathode active material layer on a current collector and punching the cathode sheet. Alternatively, cathode waste can be prepared by collecting cathodes that have defects during processing. Furthermore, cathode waste can be prepared by separating the cathode from discarded lithium-ion batteries after use.

[0053] For example, a slurry is prepared by adding N-methylpyrrolidone (NMP) to lithium cobalt oxide (e.g., LiCoO2) (LCO) or NCM-based active materials (including nickel (Ni), cobalt (Co), and manganese (Mn)), carbon-based carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder, and then coating the slurry onto a sheet current collector made of aluminum foil. The sheet is then dried in a vacuum furnace at about 120°C to produce a positive electrode sheet, and positive electrode plates of a specific size are punched out. The remaining positive electrode waste can also be prepared.

[0054] Lithium-based composite transition metal oxides are used as positive electrode active materials in lithium-ion secondary batteries, primarily utilizing lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMnO2 or LiMn2O4, etc.), lithium iron phosphate compounds (LiFePO4, etc.), or nickel lithium oxide (LiNiO2, etc.). Furthermore, as a method to improve low thermal stability while maintaining good reversible capacity of LiNiO2, nickel-manganese-based lithium composite metal oxides (in which a portion of nickel (Ni) is replaced by manganese (Mn) with good thermal stability) and NCM-based lithium composite transition metal oxides (in which a portion of nickel (Ni) is replaced by manganese (Mn) and cobalt (Co)) are used. This disclosure specifically proposes the reuse of NCM-based lithium composite transition metal oxide active materials.

[0055] As described above, the positive electrode waste has an active material layer on a current collector made of metal foil (such as aluminum foil). This active material layer is formed by coating a slurry containing active material, conductive material, binder, solvent, etc., and has a structure in which the binder connects the active material and the conductive material after the solvent evaporates. Therefore, if the binder is removed, the active material can be separated from the current collector.

[0056] Next, the cathode waste is pulverized to an appropriate size (step S20). Pulverization refers to cutting or shredding the cathode waste into suitable, easily manageable fragments. After pulverization, the cathode waste is cut into small fragments, for example, 1 cm × 1 cm. For pulverization, various dry grinding equipment (such as hand grinders, pin grinders, disc grinders, cutting grinders, and hammer grinders) can be used, or a high-speed cutter can be used.

[0057] The pulverization process can be performed taking into account the characteristics required by the equipment used in processing cathode waste and subsequent processes. For example, in cases where cathode waste is loaded and unloaded using equipment that requires continuous processing, the cathode waste must have good flowability, so it is necessary to pulverize cathode waste that is too large.

[0058] Now, the positive electrode waste is heat-treated in air (step S30).

[0059] In this disclosure, heat treatment is performed to thermally decompose the binder in the active material layer. This heat treatment can be performed at temperatures between 300°C and 650°C, which can be termed high-temperature heat treatment. Below 300°C, it is difficult to remove the binder, resulting in the current collector not being able to be separated. At temperatures equal to or greater than 650°C, the current collector melts (aluminum melting point: 660°C), leading to the phenomenon that the current collector cannot be separated.

[0060] A certain heat treatment time is maintained to allow for sufficient thermal decomposition of the binder. For example, the heat treatment time is maintained at the aforementioned heat treatment temperature for 10 minutes to 24 hours. Preferably, the heat treatment time is set to be equal to or greater than 30 minutes. The longer the heat treatment time, the longer the time for thermal decomposition of the binder, but when the heat treatment time exceeds a certain period, there is no difference in the effect of thermal decomposition. In addition, lithium reduction is undesirable. The heat treatment equipment can be various types of furnaces. For example, considering productivity, the heat treatment equipment can be a box furnace or a rotary kiln capable of continuous processing.

[0061] After heat treatment, the cathode waste can be slowly or rapidly cooled in the atmosphere.

[0062] For example, a heat treatment can be performed at 550°C for 30 minutes at a temperature rise rate of 5°C / min. This temperature rise rate can be implemented without difficulty (e.g., in a box furnace) and heating can be performed without causing thermal shock to the positive electrode waste. 550°C allows for good thermal decomposition of the binder while taking into account the melting point of the aluminum current collector. At this temperature, since thermal decomposition is insufficient to occur in less than 10 minutes of heat treatment, heat treatment should be performed for more than 10 minutes, and if possible, for more than 30 minutes.

[0063] Because the binder and conductive material in the active material layer are thermally decomposed in air through heat treatment, turning into CO2 and H2O and being removed. Since the binder is removed, the active material is separated from the current collector, and the active material to be recycled can be in powder form. Therefore, in step S30 alone, the current collector can be separated from the active material layer, and the active material in the active material layer can be recycled.

[0064] Performing the heat treatment in step S30 in air is important. If the heat treatment is performed in a reducing or inert gas atmosphere, the binder and conductive material will not be thermally decomposed, but only carbonized. When the binder and conductive material are only carbonized, the carbon component remains on the surface of the active material, which will reduce the reusability of the active material. When heat treatment is performed in air, both the binder and conductive material are almost completely removed, leaving no residue, because the carbon material in the binder or conductive material reacts with oxygen and is burned and removed as CO and CO2 gases.

[0065] Therefore, according to this disclosure, the active material is recovered in powder form, and the carbon components generated by the carbonization of the binder or conductive material do not remain on the surface.

[0066] Next, the recovered active material is washed (step S40). During washing, a cleaning solution is used. The cleaning solution can be simply water. Alternatively, the cleaning solution can be an aqueous solution of a lithium compound. In particular, an aqueous solution of a lithium compound that is alkaline in its aqueous state is preferred. This aqueous solution of the lithium compound is prepared to contain greater than 0% and equal to or less than 15% of a lithium compound, and preferably uses LiOH. The amount of LiOH is preferably equal to or less than 15%. Using excess LiOH may leave excess LiOH on the surface of the active material even after washing, which may affect the subsequent annealing process. In order to clean the surface of the active material as thoroughly as possible in the pre-annealing step, the amount added is limited to equal to or less than 15% because the addition of excess LiOH is detrimental to the process.

[0067] Washing can be performed by immersing the recovered active material in an aqueous solution of a lithium compound. After immersion, washing can be carried out within one week (preferably within one day, more preferably within one hour). If washing is performed after one week, there is a risk of capacity reduction due to excessive lithium leaching. The washing time can be determined by considering lithium dissipation. For example, in the case of compositions with a relatively high nickel content in NCM active materials, since the amount of lithium dissipation increases with the washing time, it is best not to extend the washing time as much as possible. Therefore, it is preferable to set the washing time within one day or one hour. In the case of compositions with a relatively low nickel content, if the washing time does not exceed one week, the lithium dissipation is relatively small. Therefore, it is preferable to wash within one week (preferably within one day or one hour). Washing includes immersing the active material in a cleaning solution (such as an aqueous solution of an alkaline lithium compound), stirring the active material while it is immersed, etc. It is preferable to combine stirring with immersion whenever possible. If the active material is only immersed in the cleaning solution without stirring, the washing process may proceed slowly and may result in lithium leaching. Since the process time can be minimized if agitation and soaking are performed together, it is preferable to agitate while soaking in the cleaning solution. After washing, drying can be performed simply, or the spray drying step described in another embodiment below can be performed without drying. First, when drying is performed immediately after washing, air drying can be carried out in a convection oven after filtration.

[0068] The purpose of washing is to remove LiF and metal fluorides that may be present on the surface of the recycled active material and to perform surface modification. During the heat treatment in step S30, the binder and conductive material in the active material layer are converted into CO2 and H2O, evaporated, and then removed. In this process, CO2 and H2O react with lithium on the surface of the active material to form Li2CO3 and LiOH, and fluorine (F) present in the binder (such as PVdF) reacts with the metal elements constituting the positive electrode active material to form LiF or metal fluorides. If LiF or metal fluorides remain, the battery performance will deteriorate when the active material is reused. In this disclosure, the reactants that may be generated on the surface of the recycled active material during the heat treatment in step S30 are removed by adding the washing in step S40, so that foreign matter will not remain on the surface of the recycled active material.

[0069] In particular, washing the active material with an aqueous solution of lithium compounds that is alkaline in the aqueous state is more preferable. While using an aqueous solution of sulfuric acid or hydrochloric acid instead of an aqueous solution of lithium compounds that is alkaline in the aqueous state can wash away F from the surface of the active material, the properties of the positive electrode active material will decrease because transition metals (Co and Mg) present in the active material will be eluted. The aqueous solution of lithium compounds that is alkaline in the aqueous state used in this disclosure is highly desirable because it can remove binders that may still exist in trace amounts even after the thermal decomposition in step S30, and can replenish the amount of lithium that may be eluted during the washing process without eluting transition metals and the like present in the active material.

[0070] Through step S40, in this disclosure, the LiF content on the surface of the recovered active material can be adjusted to less than 500 ppm, thereby achieving an increased capacity. Preferably, the F content can be set to be equal to or less than 100 ppm. More preferably, the F content can be set to be equal to or less than 30 ppm. As described above, through washing, LiF or lithium metal compounds formed by the decomposition of the binder are removed, thus improving the resistivity characteristics.

[0071] Next, the lithium precursor is added to the washed active material and annealed (step S50).

[0072] Although the preceding steps S30 and S40 aim to minimize lithium loss from the active material, unavoidable lithium loss may occur. This lithium loss is compensated for in step S50. A solid or liquid lithium precursor can be added. The lithium precursor used in the annealing process can be any one or more of LiOH, Li₂CO₃, LiNO₃, and Li₂O.

[0073] To mix the solid lithium precursor, drying is performed after step S40, followed by material mixing, in which case powder mixing or milling processes are used.

[0074] To mix the liquid lithium precursor, it is preferable to mix the washed active material with the lithium precursor solution after step S40 without drying, and then spray-dry the lithium precursor solution. The lithium precursor solution can be a lithium compound soluble in aqueous solution or organic solvent. The temperature of the spray-drying step can be from 100°C to 300°C. Preferably, the minimum temperature is equal to or greater than 80°C. This is because when the minimum temperature is equal to or lower than 80°C, the solution may not be completely dried. More preferably, when the minimum temperature is equal to or greater than 100°C, the solution can be completely dried.

[0075] During spray drying, the lithium precursor solution is dried immediately after spraying, coating or contacting the active material surface with the lithium precursor components. Another advantage in this regard is that, during the drying of the lithium precursor solution as a solvent, particles aggregate under capillary forces, thus adjusting the particle size distribution. In the case of positive electrode waste made from electrodes, particles on the surface may be pressed and cracked or broken during the rolling process. In particular, compared to LCO, NCM-based active materials exhibit a high degree of particle splitting during the rolling process in electrode formation. Compared to fresh active materials, recycled active materials contain many small particles, resulting in particle inhomogeneity. Specifically, NCM-based active materials comprising large particles are utilized, which are secondary granulated particles formed by aggregating primary particles with sizes ranging from tens to hundreds of nanometers. In the process of rolling positive electrodes made from such active materials to adjust the porosity in the electrode, the secondary particles split into primary granulated or smaller particles, which are larger than the secondary particles but smaller than the large particles. Since the specific surface area of ​​the active material increases with the number of particles crushed by rolling, problems may arise that affect slurry properties, electrode adhesion, and electrode characteristics when the active material is reused from the rolled electrode.

[0076] To achieve reusable levels of active materials, it is desirable that the particle size distribution not differ from that of fresh active materials. Spray drying addresses particle inhomogeneity by recovering larger particles through the aggregation of smaller particles that break down during the rolling process, and also brings the particle size closer to the initial characteristics of fresh active materials. It is particularly effective for NCM-based active materials where severe particle breakage occurs during the rolling process of previous processes.

[0077] Therefore, when using spray drying, the lithium precursor is coated onto the surface of the active material, and the active material is obtained by adjusting the particle size. Since the addition of the lithium precursor, granulation, and drying are performed in one step, the process is simplified. Because the active material particles washed in step S40 are simply mixed and dispersed in a lithium precursor solution of a certain concentration before spray drying, the advantage lies in the fact that washing in step S40 and adding the lithium precursor in step S50 can be a continuous process.

[0078] In step S50, the crystal structure of the active material is restored by annealing, thereby restoring or improving the properties of the reused active material to the level of a fresh, unused active material. Through the preceding steps S30 and S40, deformed structures may appear on the surface of the active material. For example, in an active material that is an NCM-based lithium composite transition metal oxide, a spinel structure in which nickel becomes damp and rock-salted [NiCO3·2Ni(OH)2)H2O] may form in step S40. If the battery is manufactured as is, the battery characteristics may deteriorate (e.g., capacity decrease). In this disclosure, the crystal structure is restored by step S50. For example, the active material that is an NCM-based lithium composite transition metal oxide is restored to a hexagonal structure again. Therefore, it is possible to restore or improve the initial properties of the active material to a level similar to that of a fresh active material.

[0079] Furthermore, in the case of LCO active materials, Co3O4 can be generated through thermal decomposition on the surface. If a battery containing Co3O4 is manufactured, the battery characteristics may deteriorate. In this disclosure, by restoring the crystal structure and removing Co3O4 in step S50, the initial properties of the active material can be restored or improved to a level similar to that of fresh active material.

[0080] Adding a lithium precursor before annealing can be done by comparing the proportion of lithium lost to the ratio of lithium to other metals in the original active material (i.e., fresh active material) used for the active material layer. However, even if the amount of lithium precursor added is equal to the amount lost, errors may occur due to analyses such as ICP. Furthermore, even if the capacity has been restored to the level of the original fresh positive electrode active material, if a large amount of lithium compound remains, the defect rate of the cell may increase because the lithium compound reacts with the electrolyte, generating side reactions or gases in the cell. That is, in addition to the amount of lithium lost through washing, adding an excessive amount of lithium precursor will result in unreacted lithium precursor remaining in the reused active material, which increases resistance in the active material reuse process, so it is necessary to apply an appropriate amount of lithium precursor. Most preferably, the amount of residual lithium compound is 0, but it is almost impossible to set this amount to 0. Therefore, this disclosure proposes an amount of residual lithium compound acceptable for capacity and lifetime characteristics. This disclosure proposes that the content of residual lithium compound should be at least 0.0001 wt% to 1.2 wt%.

[0081] In particular, in this disclosure, after obtaining reusable active material at the end of all processes, the amount of lithium precursor added in step S40 (when using spray drying) or S50 is determined such that the amount of residual lithium compound in the active material is 0.0001 wt% to 1.2 wt%. Since the residual lithium compound varies depending on the type of lithium precursor, it may affect the residual amount of lithium compounds such as LiOH or Li2CO3; therefore, there are no particular limitations on the type of lithium precursor or the type of residual lithium compound. However, as described above, if at least one of LiOH, Li2CO3, LiNO3, and Li2O is used as the lithium precursor, the amount of lithium precursor added is preferably added at a molar ratio equal to or less than 0.12 relative to the 1:1 molar ratio of lithium to other metals in step S40 (when using spray drying) or S50. Then, after obtaining reusable active material at the end of all processes, the amount of residual lithium compound in the active material may be equal to or less than 1.2 wt%. When using this active material to manufacture secondary batteries, it is possible to maintain lifespan characteristics while suppressing side reactions caused by the electrolyte.

[0082] As illustrated in the experimental examples described later, as a result of measuring different amounts of samples during lithium loss replenishment, lifetime characteristics deteriorate if the amount of residual lithium compound exceeds 1.2 wt%. Lower residual lithium compound levels are better. However, it is difficult to accurately set the residual lithium compound level to 0 (due to measurement equipment errors). Therefore, in this disclosure, the lower limit for residual lithium compound is considered to be equal to or greater than 0.0001 wt%. To achieve a residual lithium compound level between 0.0001 wt% and 1.2 wt%, the amount of lithium precursor added to replenish lithium loss must be appropriate. This disclosure proposes an optimized range between the content of residual lithium compound and the amount of lithium precursor added to restore the initial capacity and lifetime characteristics of the reused cathode active material.

[0083] Annealing can be performed in air at 400°C to 1000°C. The annealing temperature can be 600°C to 900°C. This temperature should be varied within a limited range depending on the type of lithium precursor. Preferably, the annealing time is set to be equal to or greater than one hour. Preferably, the annealing time is about 5 hours. If the annealing time is long, the crystal structure may be fully recovered, but even with a long annealing time, the properties of the active material will not be significantly affected. For example, the annealing time is within 15 hours. The annealing equipment can be the same as or similar to the heat treatment in step S30.

[0084] For example, when Li₂CO₃ is used as a lithium precursor, the annealing temperature is preferably 700°C to 900°C, more preferably 710°C to 780°C. This is because the melting point of Li₂CO₃ is 723°C. Most preferably, annealing is performed at 750°C. When LiOH is used as a lithium precursor, the annealing temperature is preferably 400°C to 600°C, more preferably 450°C to 480°C. This is because the melting point of LiOH is 462°C.

[0085] The annealing temperature is preferably above the melting point of the lithium precursor. However, at temperatures exceeding 1000°C, the positive electrode active material will undergo thermal decomposition, and the properties of the active material will deteriorate. Therefore, the temperature should not exceed 1000°C.

[0086] Through this step S50, reusable active materials can be obtained.

[0087] Next, as an optional step, step S60 can be performed further. In step S60, a surface coating is applied to the active material annealed in step S50.

[0088] The surface coating step may involve coating a surface with at least one of a metal, organometallic material, and carbon component in a solid or liquid manner, followed by heat treatment of the coated material at a temperature between 100°C and 1200°C. When heat treatment is performed at temperatures exceeding 1200°C, the properties may degrade due to the thermal decomposition of the positive electrode active material. In the surface coating step, the coating process, whether solid or liquid, can utilize methods such as mixing, milling, spray drying, and grinding.

[0089] By surface coating, a heterogeneous metal forms a surface protective layer. When the molar ratio of lithium to other metals in the positive electrode active material is 1:1 by replenishing the lost lithium, if the lithium in the active material reacts with the surface coating material and the molar ratio of lithium to other metals in the positive electrode active material drops to less than 1:1, 100% capacity performance may not be achieved. Therefore, not only should insufficient lithium be added in the preceding step S50 to make the molar ratio of lithium to other metals in the positive electrode active material 1:1, but also excess lithium should be added to include a greater amount of lithium than other metals in the positive electrode active material. Thus, during the surface coating process, a surface protective layer can be formed with a 1:1 molar ratio of lithium to other metals in the positive electrode active material. Therefore, even during the formation of the surface protective layer, it is preferable to further add lithium, in addition to simply adding the lost lithium during the process, to use it as a material for the surface protective layer. For example, not only is the molar ratio of lithium to other metals in the positive electrode active material 1:1, but an excess of lithium is added, resulting in a higher lithium content (0.0001 to 0.12 molar ratio) compared to other metals in the positive electrode active material. Thus, during surface coating, the molar ratio of lithium to other metals in the positive electrode active material is 1:1, and a surface protective layer can be formed. When no surface protective layer is formed, an excess of lithium is not required. As described above, the amount of lithium precursor added should be determined considering whether a protective layer is formed. As described above, after obtaining reusable active material at the end of all processes, the amount of lithium precursor added should be determined so that the amount of residual lithium compound in the active material is between 0.0001 wt% and 1.2 wt%.

[0090] Specifically, when metal oxides such as B, W, and BW are coated onto the active material and then heat-treated, a lithium boron oxide layer can be formed on the surface of the active material as a surface protective layer. The additional lithium added in step S50 reacts with the metal oxides such as B, W, and BW in step S60, and the molar ratio of lithium to other metals in the positive electrode active material does not decrease to less than 1:1, thus preventing capacity reduction.

[0091] The reusable active material obtained by the above method can be represented by the following chemical formula 1.

[0092] [Chemical Formula 1]

[0093] Li a Ni x Mn y Co z M w O 2+ δ

[0094] (In the above Chemical Formula 1, M includes at least one selected from the group consisting of B, W, Al, Ti, and Mg, 1 < a ≤ 1.1, 0 ≤ x < 0.95, 0 ≤ y < 0.8, 0 ≤ z < 1.0, 0 ≤ w ≤ 0.1, -0.02 ≤ δ ≤ 0.02, and x + y + z + w = 1.)

[0095] The F content of the reusable active material is equal to or lower than 100 ppm. According to the present disclosure, since an active material with a reduced F content can be recovered, if the active material with a reduced F content is reused as an active material, good resistance characteristics and capacity characteristics can be achieved.

[0096] In particular, the method for reusing the active material according to the present disclosure has been optimized for NCM-based active materials. This is because, compared with LCO-based active materials, NCM-based active materials have a larger specific surface area, since NCM-based active materials include secondary particles in which primary particles are aggregated, and it is difficult to control the amount of residual lithium compounds.

[0097] As described above, according to the present disclosure, during the heat treatment process in step S30, the active material and the current collector are separated. By optimizing the temperature, time, etc. of the heat treatment process, the reduction of lithium in the active material after heat treatment can be minimized. Thus, the amount of lithium precursor to be added later can be minimized.

[0098] During the washing process in step S40, LiF or metal fluoride is removed. Washing is safe and inexpensive, and can remove LiF or metal fluoride without loss of other elements, and prevent elution of transition metals, etc. In particular, by optimizing the conditions of the cleaning solution, washing time, etc., the reduction of lithium in the active material can be minimized. Thus, the amount of lithium precursor to be added later can be minimized. If an aqueous solution of a lithium compound that is alkaline in an aqueous solution is used as the cleaning solution during the washing process, there is also an advantage of compensating for the lithium loss that occurs during this process.

[0099] As described above, according to this disclosure, lithium loss in the active material can be minimized in the heat treatment process for separating the current collector and the washing process (e.g., residue removal) for surface modification. By optimizing the heat treatment process, lithium loss can be minimized, as can the amount of current collector reacting with the active material or precipitated through heat treatment. Furthermore, lithium loss in the active material can be minimized while removing resistive LiF that might act if remaining. Since lithium loss is minimized during the process of obtaining the reusable active material, the composition of the fresh active material is not significantly different from that of the reusable active material. Insufficient lithium is supplemented by an additional lithium precursor addition process. In this regard, the amount of lithium precursor added is determined such that the amount of residual lithium compounds in the reusable active material is between 0.0001 wt% and 1.2 wt%, thus maintaining good lifetime and capacity characteristics.

[0100] If spray drying is used to add lithium precursors, the active material to be reused can be re-granulated through spray drying, improving particle size and specific surface area even if the particles are broken due to rolling in the previous process. Furthermore, if the cleaned active material is mixed with the lithium precursor solution and then spray-dried, the lithium precursor can be added while the active material is being re-granulated, simplifying the process and providing the advantage of forming a continuous process with the preceding washing step.

[0101] The annealing step following the addition of lithium precursors also has the advantages of being safe and inexpensive; effectively removing Co3O4; and restoring the cell properties of reusable active materials by restoring the crystal structure (i.e., increasing crystallinity).

[0102] The reusable active material obtained according to this disclosure can have a particle size distribution similar to that of fresh active material, thus potentially eliminating the need for separate treatment to adjust the particle size distribution. Since no carbon components generated through the carbonization of binders or conductive materials remain on the surface, steps such as carbon removal are unnecessary. Therefore, through… Figure 2 The active material obtained by the above method can be reused as is without additional processing and used to manufacture the positive electrode.

[0103] The reused active material can also be used 100% as is without adjusting the composition, or the reused active material can be mixed with fresh active material, and the reused active material can be mixed with conductive material, binder and solvent to make and use a slurry.

[0104] The experimental examples of this disclosure will be described in detail below.

[0105] <Experimental Example>

[0106] Experimental Example 1: After each step of the positive electrode active material recycling method, the molar ratio of lithium to other metals was analyzed. Erbi

[0107] The following methods were used to prepare various positive electrode active materials. The molar ratios of lithium to other metals were measured and summarized in Table 1 below.

[0108] Sample 1: Utilizing fresh NCM-based lithium composite transition metal oxides instead of recycled active materials.

[0109] Sample 2: The positive electrode waste to be discarded after preparing the punched positive electrode plate with NCM-based lithium composite transition metal oxide active material was heat-treated in air at 500°C for 5 hours, and then the active material was collected. That is, in the above-disclosed method for reusing active materials, only the heat treatment in step S30 is performed to remove the binder and conductive material, separate the aluminum current collector, and collect the NCM-based lithium composite transition metal oxide active material. At this time, the positive electrode plate is made from a slurry, which is prepared as follows: 96.25 wt% of positive electrode active material, 1.5 wt% of carbon black as conductive material, and 2.25 wt% of PVdF as binder (resin content of 9.8 wt% relative to PVdF 1100) are weighed and mixed with NMP.

[0110] Sample 3: Sample 2 was further washed for 10 minutes according to the washing step (S40) of the active material reuse method of this disclosure. An aqueous solution containing LiOH was used as the cleaning solution.

[0111] Sample 4: Sample 3 was annealed only in step S50 without the addition of lithium precursor. Annealing was performed in air at 750°C for 15 hours.

[0112] Sample 5: Sample 3 was annealed in air at 750°C for 15 hours by adding Li2CO3 as a lithium precursor at a molar ratio of 0.05.

[0113] Sample 6: Sample 3 was annealed in air at 750°C for 15 hours by adding Li2CO3 as a lithium precursor at a molar ratio of 0.08.

[0114] Sample 7: Sample 3 was annealed in air at 750°C for 15 hours by adding Li2CO3 as a lithium precursor at a molar ratio of 0.09.

[0115] Sample 8: Sample 3 was annealed in air at 750°C for 15 hours by adding Li2CO3 as a lithium precursor at a molar ratio of 0.1.

[0116] Sample 9: Sample 3 was annealed in air at 750°C for 15 hours by adding Li2CO3 as a lithium precursor at a molar ratio of 0.11.

[0117] Sample 10: Sample 3 was annealed only in step S50 without the addition of lithium precursor. Annealing was performed in air at 750°C for 5 hours, which was shorter than that of Sample 4.

[0118] Table 1

[0119] Sample number Molar ratio of lithium to other metals 1 1.02 2 1 3 0.93 4 0.92 5 0.97 6 1 7 1.01 8 1.01 9 1.05 10 0.92

[0120] Referring to Table 1, it can be seen that, compared to Sample 1, the molar ratio of lithium to other metals gradually decreases with each step of the active material recycling process. That is, even with only heat treatment as in Sample 2, lithium is lost, and the loss is even greater when washing is performed as in Sample 3. In particular, Sample 4, annealed at 750°C for 15 hours without the addition of lithium precursors, shows a molar ratio decrease of approximately 0.1 compared to Sample 1. Therefore, lithium is lost during heat treatment, washing, and annealing, thus requiring the addition of lithium.

[0121] From Sample 5 to Sample 9, the amount of lithium precursor added gradually increased, and the molar ratio of lithium to other metals gradually increased. To compensate for the approximately 0.1 decrease in the molar ratio of Sample 4 compared to Sample 1, the molar ratio of lithium to other metals was 1.01 after evaluating Sample 7, which was annealed with Li₂CO₃ added at a molar ratio of 0.09, and Sample 8, which was annealed with Li₂CO₃ added at a molar ratio of 0.1. Considering the instrument measurement error of ±0.02 for ICP, the molar ratio of lithium to other metals in Samples 7 and 8 may have a range of 0.99 to 1.03, which was determined to be equivalent to Sample 1. Samples 5 and 6, which added Li₂CO₃ at a molar ratio less than 0.09, had a smaller molar ratio of lithium to other metals than Sample 1. Therefore, it can be confirmed that when a lithium precursor (sample 8) is added, similar to the reduction ratio demonstrated in sample 4 (where the molar ratio of sample 1 to sample 4 is 0.1) after annealing without the addition of the lithium precursor, the molar ratio of lithium to other metals can be increased to the same level as in sample 1. Furthermore, compared to sample 3, sample 10 shows a decrease in annealing time after washing, but it can be seen that the molar ratio of lithium to other metals remains unchanged. That is, it is confirmed that even annealing times equal to or greater than 5 hours have no significant effect on the reduction of lithium.

[0122] The above experiments confirm that adding a lithium precursor is preferable, especially under the conditions of this experimental example. If the lithium precursor is added at a molar ratio equal to or greater than 0.09, the lithium content of the lithium precursor may be similar to that of the fresh active material. It can be seen that the amount of lithium precursor required to achieve a lithium content similar to that of the fresh active material may vary depending on the experimental conditions.

[0123] Experiment Example 2: Experiment with the addition of lithium precursor and boron coating

[0124] The following methods were used to prepare various positive electrode active materials, manufacture positive electrodes, and then manufacture battery cells (button half-cells, CHC), and evaluate their electrochemical performance.

[0125] Comparative Example 1: Using fresh NCM-based lithium composite transition metal oxides, such as Sample 1.

[0126] Implementation Method 1: Continue as with Sample 8 until annealing, but set the annealing time to 5 hours, which is shorter than that of Sample 8. That is, the positive electrode waste is heat-treated in air at 550°C for 30 minutes, washed for 10 minutes, and then annealed at 750°C for 5 hours by adding Li2CO3 as a lithium precursor at a molar ratio of 0.1.

[0127] Implementation Method 2: In addition to Implementation Method 1, the following was also carried out. Figure 2 The optional step S60 is a process for restoring the protective layer on the surface of the active material. This is achieved by adding 500 ppm of boron and heat-treating at 300°C for 5 hours, thus restoring the protective layer on the surface of the active material.

[0128] Implementation method 3: Same as implementation method 2, except that the amount of lithium precursor added is 0.11 molar ratio, which is greater than the 0.1 molar ratio in implementation method 2.

[0129] Implementation method 4: Same as implementation method 2, except that the amount of lithium precursor added is 0.12 molar ratio, which is greater than the molar ratio of 0.1 in implementation method 2.

[0130] Comparative Example 2: Same as Embodiment 2, except that the amount of lithium precursor added is 0.13 molar ratio, which is greater than the 0.1 molar ratio in Embodiment 2.

[0131] The positive electrode plate is made from a slurry prepared as follows: 96.25 wt% of the positive electrode active material recovered or prepared in the above embodiments and comparative examples, 1.5 wt% of carbon black as a conductive material, and 2.25 wt% of PVdF as a binder (resin content of 9.8 wt% relative to PVdF 1100) are weighed and mixed with NMP.

[0132] Figures 3 to 5 The cell evaluation results using the active materials of the embodiments and comparative examples are shown.

[0133] Figure 3 The initial charge / discharge characteristics are shown. Figure 4 yes Figure 3 A magnified view of a portion of the time period. Figure 3 and Figure 4 In the diagram, the horizontal axis represents capacity, and the vertical axis represents voltage.

[0134] In Experiment 1 above, it was confirmed that the molar ratio of lithium to other metals in the lithium precursor to be added to Sample 8 can be equivalent to the molar ratio of fresh active material. In Embodiment 1, the lithium precursor of Sample 8 was added, so it can be predicted that the capacity characteristics of Embodiment 1 will be similar to those of Comparative Example 1 (fresh active material). As an experimental result, when measuring the initial charge / discharge capacity characteristics, when the lithium precursor was added at a molar ratio equal to or greater than 0.1 as in Embodiments 1 to 4, the capacity was equal to or greater than the capacity of Comparative Example 1 (fresh active material), which is consistent with the prediction. However, in Comparative Example 2, where the lithium precursor was added at a molar ratio equal to or greater than 0.13, the capacity decreased slightly, thus it was found that adding a certain amount or more of the lithium precursor increases the resistance and has an adverse effect on the capacity characteristics.

[0135] Figure 5 The C-rate characteristics are shown. The rate characteristics are evaluated by assessing the capacity based on the number of cycle repetitions at different currents. The equipment used for evaluation is a common charge / discharge test apparatus used in a laboratory. There is no bias based on the measurement equipment or method. Figure 3 In the chart, the horizontal axis represents the number of cycles, and the vertical axis represents the capacity. The voltage was set from 3V to 4.3V, and the initial charge / discharge was performed at 0.1C / 0.1C. The electrolyte constituting the cell was utilized; it was carbonate-based, with a ethylene carbonate (EC) to ethyl carbonate (EMC) ratio of 3:7, and included some additives.

[0136] like Figure 5 As shown, the C-rate characteristics are similar in Embodiments 2 to 4, which have undergone boron coating. Table 2 summarizes the initial charge / discharge capacity characteristics of Comparative Example 1 and Embodiments 2 and 4.

[0137] Table 2

[0138]

[0139] Figure 5 The results shown indicate that the capacity of Embodiment 4 decreases compared to Embodiment 2 as the number of cycles increases. While the capacity decrease in Embodiment 4 is acceptable, it is not preferable in terms of capacity reduction if the amount of lithium precursor added is increased compared to Embodiment 4.

[0140] Therefore, it has been confirmed that when considering capacity characteristics, it is preferable to add the lithium precursor at a molar ratio equal to or less than 0.13, while when considering cycle characteristics, it is preferable to add the lithium precursor at a molar ratio equal to or less than 0.12. The upper limit for the addition of lithium precursor may vary depending on experimental conditions. Nevertheless, those skilled in the art will understand that the amount of lithium precursor added without degrading capacity and cycle characteristics can be determined according to the recommendations of this disclosure.

[0141] Experimental Example 3: Correlation between the amount of lithium precursor added and pH titration

[0142] Regarding Comparative Example 1 and Embodiments 2 and 4, 5 grams of each active material were dispersed in 100 ml of distilled water and mixed at 300 rpm for 5 minutes, followed by filtration of the active material. When titrating the filtered solution with 0.1 M HCl solution, the pH value and the amounts of LiOH and Li₂CO₃ dissolved from the active material were measured.

[0143] Figure 6 This is a graph showing the pH titration results. Table 3 shows the data obtained by calculating the amount of LiOH and Li2CO3 remaining in the active material based on the measured pH values.

[0144] Table 3

[0145]

[0146] The total amount of residual lithium compounds is obtained by adding the residual amounts of LiOH and Li2CO3.

[0147] It can be seen that the residual lithium compound in Embodiment 2 is equal to or less than that in Comparative Example 1, while the residual amount of Li2CO3 in Embodiment 4 is 2 to 3 times that in Comparative Example 1 or Example 2. In this experiment, since Li2CO3 was used as a lithium precursor, the residual amount of Li2CO3 was large, and the content of residual lithium compound (lithium precursor) may vary depending on the type of lithium precursor used.

[0148] Experiments 1 and 2 above confirmed that good capacity and lifetime characteristics are achieved when the amount of lithium precursor added is equal to or less than a molar ratio of 0.12. In Experiment 3, the content of residual lithium compounds was confirmed based on the amount of lithium precursor added. When the amount of lithium precursor added was a molar ratio of 0.12, the amount of residual lithium compounds was calculated to be 1.212 wt%, therefore, the upper limit of residual lithium compounds was determined to be 1.2 wt%.

[0149] Therefore, when lithium is added in an amount equal to or less than 0.12 relative to the 1:1 molar ratio of lithium to other metals, the amount of residual lithium compound can be equal to or less than 1.2 wt%. An amount of residual lithium compound greater than 1.2 wt% is not preferred because the cycle characteristics deteriorate and the gas generation rate increases, as in embodiment 4.

[0150] Since the residual lithium compound varies depending on the type of lithium precursor, it may affect the residual amount of LiOH or Li₂CO₃. Therefore, there are no particular limitations on the type of lithium precursor or the type of residual lithium compound. However, as mentioned above, if at least one of LiOH, Li₂CO₃, LiNO₃, and Li₂O is used as the lithium precursor, since equivalent properties to fresh active material can be expected based on the results of Experimental Examples 1 and 2, it is preferable to further add lithium precursor in an amount of 0.09 to 0.12 molar ratio relative to a 1:1 molar ratio of lithium to other metals. Then, after obtaining reusable active material at the end of all processes, as shown in Experimental Example 3, the amount of residual lithium compound in the active material can be from 0.473 wt% to 1.2 wt%.

[0151] In these experimental examples, the process continued up to a surface coating step using 500 ppm boron. If the amount of boron is further increased, the amount of lithium added with the boron will increase, and therefore the amount of residual lithium compound may be further reduced. Therefore, the lower limit for the amount of residual lithium compound is controlled to be equal to or greater than 0.0001 wt%, which is considered the measurable limit.

[0152] As described above, the amount of lithium precursor added should be appropriate in order to keep the amount of residual lithium compound from 0.0001 wt% to 1.2 wt%. In the experimental examples of this disclosure, lithium precursors can be added at a further molar ratio of 0.0001 to 0.13 relative to a 1:1 molar ratio of lithium to other metals. It has been shown that the lifetime characteristics deteriorate relative to a molar ratio of 0.12, and when lithium is added (converted) at a molar ratio of 0.12, the amount of residual lithium compound is 1.2 wt%. Since the appropriate molar ratio varies depending on the type of lithium precursor, as in this disclosure, it is preferable to manage the amount of lithium precursor added by limiting the amount of residual lithium compound in the final product, rather than setting the amount of lithium precursor added within a general range.

[0153] Additional Experimental Examples

[0154] 1) To determine the amount of residual LiF in the active materials recovered from Samples 2 and 3 of Experimental Example 1, F was detected and analyzed by ICP. In Sample 2, the detected F was 1450 mg / kg, while in Sample 3, the detected F was ND, i.e., equal to or less than 30 ppm. Compared with Sample 2, the F content in the recovered positive electrode active material in Sample 3 was significantly reduced, which confirms that LiF was completely dissolved in the lithium compound aqueous solution by washing and removed to a degree that LiF may not be detectable by ICP. Therefore, it can be seen that LiF is well removed by step S40 of the active material reuse method according to this disclosure.

[0155] 2) The rate characteristics were also evaluated using the active materials recovered from samples 2 to 4 and sample 7 of Experimental Example 1.

[0156] The active material before surface modification in step S40 is sample 2, and the active material after surface modification is sample 3. Comparing the two active materials, it was confirmed that the electrode capacity in sample 3 decreased rapidly. This is because, as described above, the nickel in the NCM-based active material is hydrated and halogenated, resulting in a decrease in capacity.

[0157] In another sample that was annealed (at 750°C for 15 hours) without surface modification, there was almost no increase in capacity compared to sample 2. This is because LiF remains on the surface of the active material without surface modification.

[0158] Sample 4 is a sample that underwent surface modification and annealing after the first heat treatment. An increase in capacity was confirmed in Sample 4. This is because, although the capacity decreased after the surface modification step, similar to Sample 3, in Sample 4, after removing LiF through surface modification, the nickel rock salt was reduced by annealing, and the structure was restored to a hexagonal crystal.

[0159] The capacity improvement of Sample 7 compared to Sample 4 was also confirmed. Sample 7 was obtained by adding a lithium precursor at a molar ratio of 0.09 during the annealing process, compared to Sample 4. It can be seen that by adding the lithium precursor as described above to compensate for the lithium loss in the preceding steps, the capacity was improved. It was also confirmed that the capacity improvement of Sample 7 showed an equivalent effect to Comparative Example 1 of Experimental Example 2 above.

[0160] As described above, according to this disclosure, active materials can be recovered from cathode waste for direct reuse. It is safe because no toxic and explosive solvents such as NMP, DMC, acetone, and methanol are used, and it is suitable for large-scale production due to the use of simple and safe methods such as heat treatment, cleaning and drying, and annealing.

[0161] 3) For the positive electrode active materials recovered or prepared from Experimental Examples 1 and 2, the contents of B and W as specific elements were also analyzed by ICP analysis.

[0162] The fresh active material used in this experiment was Sample 1 or Comparative Example 1, which, as analyzed, further included B and W, with B content of 500 mg / kg and W content of 3100 mg / kg. As measured in Sample 2, the B content decreased to 200 mg / kg, and the W content decreased to 2700 mg / kg. In Samples 3, 4, and 7, the B content was ND, as if B was almost completely removed, and the W content was significantly reduced to 200 mg / kg. Therefore, depending on the type of active material initially used, which may be susceptible to heat and moisture, specific elements may be lost during the process, especially during surface modification processes via cleaning, where specific elements may be completely removed or remain in small amounts. It is possible that, as in Sample 7, simply proceeding to the annealing step is insufficient to fully restore the properties. In such cases, it is preferable to perform the additional surface coating step proposed in this disclosure. In the case of this experimental example, the surface coating step is coating B and W. In Embodiment 2, B coating is performed compared to Embodiment 1. Therefore, surface coating can act as a protective layer on the surface of the positive electrode active material. Surface coating can also be a process to supplement specific deficient elements, while simultaneously rebuilding the protective layer in the fresh active material. In Embodiment 2, a lithium precursor can be added at a further lithium addition amount of 0.1 molar ratio relative to the 1:1 molar ratio of lithium to other metals. The lithium precursor added at a further molar ratio of 0.1 reacts with B to form a protective layer, and some remains. As demonstrated in Experimental Example 3, the amount of residual lithium compound in Embodiment 2 is 0.473 wt%, and the capacity of the residual lithium compound is equal to or greater than that of Comparative Example 1 and less than 1.2 wt%. 1.2 wt% is the upper limit of residual lithium compound with good lifetime characteristics, and therefore it is considered very ideal.

[0163] This disclosure has been described in detail. However, it should be understood that while the detailed description and specific embodiments illustrate preferred embodiments of this disclosure, they are given by way of illustration only, as various changes and modifications within the scope of this disclosure will be apparent to those skilled in the art based on this detailed description.

Claims

1. A method for recycling a positive electrode active material, the method for recycling the positive electrode active material comprising the following steps: (a) By heat-treating a positive electrode waste including a lithium composite transition metal oxide positive electrode active material layer in air, thermally decomposing a binder and a conductive material in the active material layer, separating the current collector from the active material layer, and recovering the active material in the active material layer; (b) Washing the recovered active material with a cleaning liquid; (c) Adding a lithium precursor to the washed active material and annealing the active material; and (d) Obtaining a recyclable active material by selectively performing a surface coating on the annealed active material, wherein, in step (c), the addition amount of the lithium precursor is determined such that the amount of residual lithium compound in the recyclable active material is 0.0001 wt% to 1.2 wt%, and wherein the cleaning liquid is an aqueous solution of a lithium compound that is alkaline in an aqueous solution state.

2. The method for reusing positive electrode active materials according to claim 1, wherein, The heat treatment is carried out at 300 °C to 650 °C for 10 minutes to 24 hours.

3. The method for reusing positive electrode active materials according to claim 1, wherein, The aqueous solution of the lithium compound is prepared to contain more than 0% and equal to or less than 15% of the lithium compound and is used for washing within one week.

4. The method for reusing positive electrode active materials according to claim 1, wherein, The washing is carried out by immersing the recovered active material in the aqueous solution of the lithium compound and simultaneously stirring the recovered active material.

5. The method for reusing positive electrode active materials according to claim 1, wherein, The lithium precursor includes at least one of LiOH, Li2CO3, and Li2O.

6. The method for reusing the positive electrode active material according to claim 1, wherein, The lithium precursor is added with an additional lithium addition amount in a molar ratio of 0.09 to 0.12 relative to a 1:1 molar ratio of lithium to other metals.

7. The method for reusing positive electrode active materials according to claim 1, wherein, In the case of not performing drying after the washing, the lithium precursor is added in step (c) by mixing the washed active material in a lithium precursor solution and spray-drying the active material.

8. The method for reusing the positive electrode active material according to claim 7, wherein, The temperature of the spray drying is 100 °C to 300 °C.

9. The method for reusing positive electrode active materials according to claim 1, wherein, The annealing is carried out in air at 400 °C to 1000 °C.

10. The method for reusing positive electrode active materials according to claim 1, wherein, The temperature of the annealing exceeds the melting point of the lithium precursor.

11. The method for reusing positive electrode active materials according to claim 1, wherein, The active material in the active material layer is recovered in powder form, and the carbon component generated by the carbonization of the binder or the conductive material does not remain on the surface.

12. The method for reusing positive electrode active materials according to claim 2, wherein, Performing the surface coating includes coating at least one of a metal, an organometal, and a carbon component on the surface by a solid or liquid method, and then performing a heat treatment at 100 °C to 1200 °C.

13. The method for reusing the positive electrode active material according to claim 1, wherein, The recyclable active material is represented by the following Chemical Formula 1, Li a Ni x Mr y Co z M w O 2+δ (In the above Chemical Formula 1, M includes at least one selected from the group consisting of B, W, Al, Ti, and Mg, 1 < a ≤ 1.1, 0 ≤ x < 0.95, 0 ≤ y < 0.8, 0 ≤ z < 1.0, 0 ≤ w ≤ 0.1, -0.02 ≤ δ ≤ 0.02, and x + y + z + w = 1).

14. The method for reusing positive electrode active materials according to claim 1, wherein, The recyclable active material includes fluorine (F) with a content equal to or less than 100 ppm.

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